Briefly, fiber optic technology relates to the transmission of light through a light conducting material such as optical glass, fused silica, and certain plastics. The choice of a particular material depends on the intended use of the light transmission system, and takes into consideration the properties of the fiber including its refractive index, light transmittance, as well as thermal and chemical characteristics. The size (e.g., diameter and length) and configuration of the fiber optic device is also selected based on the intended use. Devices derived from light conducting materials having relatively large diameters are referred to as light pipes. In contrast, thin filaments having significantly smaller radii (e.g., from 100 to 3,000 micrometers, .mu.m) are commonly referred to as optical fibers.
Known systems are designed so that light travels through an optical fiber by total internal reflection. Light entering the optical fiber is retained by and guided through the fiber, ultimately exiting at the other end. Basically, as light is propagated through the fiber, rather than escaping from the fiber, light striking the surface of the fiber is reflected. The extent of light reflection at the fiber surface, and conversely the loss of light from the fiber due to refraction, is a function of the indices of refraction of the fiber and its surrounding medium. For example, light incident on a high-to-low refractive index boundary (such as the interface between an optical fiber and air) at any angle greater than the critical angle is 100% reflected at the interface. Typical refractive indices for optical fibers range from about 1.2 to about 1.8, whereas the refractive index of air is 1.0003. The critical angle is a property of the light conducting material and defined as the smallest angle with the normal to the boundary at which total internal reflection occurs. Thus, for light propagated through a high-index material and striking the walls at greater than the critical angle, no refractive loss of light from the fiber occurs and the light is channeled through the fiber by total internal reflection.
In practice, despite the highly efficient transmission of light by total internal reflection in optical fibers, some light loss from the fiber inevitably occurs. Light losses may include, for example, refractive loss resulting from incident light striking the fiber walls at less than the critical angle. Additional losses may also be attributed to optical impurities present within the fiber, which may scatter or absorb light traveling through the fiber.
In addition to the light losses noted above, the attenuation of light intensity through an optical fiber may result from engagement of a fiber with a medium having a refractive index approaching the index of the fiber. For example, when an optical fiber is engaged by a liquid having a relatively high refractive index, such as water (refractive index 1.33) or gasoline (refractive index 1.38), light loss from the fiber may occur.
Using these principles, the detection of liquid levels by fiber optic sensing is well known. Numerous fiber optic devices and methods exist for the measurement of fluid levels, such as fuel in a storage tank. Many of these devices and methods take advantage of the attenuation of light intensity through a light-conducting medium by refractive loss as a consequence of engaging the optical fiber with a refractive medium such as a liquid.
Relying on this operating principal, U.S. Pat. No. 4,187,025 to Harmer discloses a light guide having alternating curvatures (e.g., S- or W-shaped light guides) to produce a light signal corresponding to the refractive index of a liquid in contact with the guide. When immersed in a liquid, the alternating curvatures of the light guide provide refractive passage of an amount of light that is variable and depends on the refractive index of the liquid. For these curvatures, the ratio of radius of curvature to the radius of the cylindrical light guide core is preferably between 3 and 5. The alternating curvature configuration of the device provides for enhanced sensitivity compared to a curved section bent in a single direction, such as the U-shaped device disclosed in U.S. Pat. No. 4,082,959 to Nakashima et al.
U.S. Pat. No. 4,287,427 to Scifres discloses several configurations of a fiber optical light guide useful for detecting liquids based on the various liquids' indices of refraction. The disclosed configurations include U-shaped and coiled light guides which, on immersion in a liquid, lose transmitted light as a function of the refractive index of the liquid.
A fiber optic detection system having a single fiber optic element in a U-shaped configuration and having a light variable loop section is disclosed in U.S. Pat. No. 5,362,971 to McMahon et al. Light transmitted through the light variable loop section escapes from the fiber when the loop section is contacted with a medium. For this system, the higher the index of refraction of the medium, the greater the amount of escaping light.
The devices noted above all share the characteristic of transmitting light through a smooth and continuous optical light guide. Optical guides having distinct reflective and refractive surfaces have also been employed to measure liquid levels. U.S. Pat. No. 3,995,169 to Odden discloses a U-shaped light pipe having planar internal reflecting surfaces positioned at both bends of the pipe. The planar surfaces act to reflect light from one arm of the pipe to the other arm without appreciable light loss when the refractive index of the surrounding medium is less than that of the light pipe. However, when the reflecting surfaces are immersed in a liquid, the planar surfaces become refractive surfaces and provide for the refraction of light from the light pipe to the surrounding liquid.
The use of reflective/refractive surfaces in optical devices to measure the presence of a liquid in contact with the surface, such as described above, is well known. Many of these optical devices include such surfaces present in conical configurations. In these optical devices, light is transmitted to the conical tip of the light guide where light is either: a) reflected across the tip and then returned via the light guide to a photodetector, when the conical tip of the guide is not in contact with a refracting medium such as a liquid; or b) refracted into the surrounding medium when the cone is immersed in a liquid. See, e.g., U.S. Pat. No. 3,384,885 to Forbush, U.S. Pat. No. 3,535,933 to Pliml, U.S. Pat. No. 3,553,666 to Melone, U.S. Pat. No. 3,683,196 to Obenhaus, and U.S. Pat. No. 3,8321,235 to Bouton et al.
In addition to the use of refractive surfaces in cone-shaped optical devices, refractive surfaces have also been incorporated into fiber optic sensors. A fiber optic probe system sensor having a refracting surface is disclosed in U.S. Pat. Nos. 4,851,817 and 5,005,005 to Brossia et al. The disclosed optical fiber has a U-shaped configuration similar to those noted above for Scifres and McMahon. However, in contrast to the above-noted optical fibers, the optical fiber in Brossia provides a sensor portion having a rough, abraded refracting surface in the light path. The abraded refracting surface provides an opportunity for light to refract from the fiber and into the sensed medium. The more abraded the fiber, the more opportunities for energy passing through the fiber to interact with the sensed medium.
The devices noted above use refractive light loss from a light guide to sense the presence of a refractive medium in contact with the guide. However, in addition to light loss from an optical fiber through refraction, light loss from a fiber may also occur through evanescent wave losses.
As used herein, the term "evanescent wave" refers to electromagnetic radiation that results from the propagation of light through a light-conducting medium, and that is present outside of the light-conducting medium. When light is transmitted through a high index of refraction medium the evanescent wave (or field) is produced in the adjacent lower index of refraction material and has intensity only within a fractional wavelength distance from the interface between the two mediums. The intensity of the evanescent wave decreases exponentially with distance from the fiber core (i.e., E=E.sub.o e.sup.-.alpha.r where E is the intensity of the evanescent wave, E.sub.o is the light intensity in the optical fiber, and .alpha. relates to the differences in the index of refraction of the two mediums, and r is the distance from the fiber core). The presence in the field of a medium that absorbs light of the wavelength of the transmitted light will result in light loss from the fiber.
Just as for refractive light loss from optical fibers, sensors and related methods have been devised to exploit evanescent wave loss from optical fibers as a means for measuring or monitoring, for example, liquid levels in a tank or reservoir. For example, U.S. Pat. No. 4,287,427 to Scifres describes a liquid-level monitor including a fiber optic light guide having a fiber consisting of a core material surrounded by a cladding material. While most of the guided light is confined to the core, a small amount of light is present in the cladding. If the cladding is removed or is sufficiently thin, the evanescent wave in the thin cladding or, in the absence of cladding, near the outer edge of the core interacts with the surrounding medium. Several configurations of the fiber optic light guide are disclosed including partially and fully cladded, coiled and U-shaped fibers. For this device, evanescent wave loss from the fiber occurs primarily when the wavelength of the guided light matches the absorbance wavelengths of the surrounding medium.
A fiber optic evanescent wave sensor system is described in U.S. Pat. No. 5,291,032 to Vali et al. The sensor system includes a light source, detector, and a cladded optical fiber having a reflector at one end. In the system, infrared light matching the absorbance wavelengths of hydrocarbons, such as those present in fuels, is transmitted into the fiber. The cladding layer is sufficiently thin to permit evanescent wave light loss to the environment. When the fiber is immersed in an absorbing medium, evanescent wave loss occurs as a function of the length of the fiber immersed in the liquid. The amount of light returned to the detector by reflection from the end of the fiber is indicative of the depth of fiber immersion and amount of liquid present.
Accordingly, despite the number and variety of optical fiber sensors and methods for sensing various environmental parameters, there remains a need in the art for improved optical sensors that are highly sensitive, low cost, durable, compact, portable and suitable for field installation. The present invention seeks to fulfill these needs and provides further related advantages.